Multimode fiber is used in high-speed data networks together with high-speed sources that typically use transversely multimode vertical cavity surface emitting lasers “VCSELs”. Historically, bit rates have been limited to 10 Gigabits per second (Gbps) for an Ethernet link and 14 Gbps for a Fiber Channel link, and reach has been limited to less than 400 m, more often less than 100 m.
In order to satisfy constraints of large data centers and comply with the ever increasing demand for bandwidth, such data networks have to cope with chromatic dispersion impairments. Chromatic dispersion impairments depend on the spectral width of the source that is generally transversely multimode, and the larger the spectral width, the larger the chromatic dispersion impairments.
Optical links having a length exceeding 400 m (e.g., 550 m) and working at least at 10 Gbps for spectrally wide VCSELs, and VCSELs operating at 25 or 28 Gbps (which are not yet commercialized), will be more affected by chromatic dispersion than optical links having a short length of about 300 m and working at a bit rate of 10 Gbps. The longer the length of the optical link and the higher the data bit rate, the more the optical link is affected by chromatic dispersion. For optical links with a long length and/or high bit rate, it becomes necessary to mitigate chromatic dispersion. Several options for mitigating chromatic dispersion include using spectrally narrow sources and compensating for chromatic dispersion.
Since spectrally narrow sources (e.g., single or quasi-single mode sources) are costly, compensating for chromatic dispersion within the multimode fiber itself is a more cost-effective approach. Compensation of chromatic dispersion in multimode fibers is partly based on the interaction between the chromatic and the modal dispersions. It has been shown that for perfect alpha (α) profiles, multimode fibers exhibiting an alpha slightly below the optimum for a given operating wavelength partially compensate for the chromatic dispersion inherent to the spectral width of typical transversely multimode light sources.
In practice, however, the refractive index profile is not accurately controllable enough to produce only fibers with such a desired feature. As a consequence, a differential mode delay (DMD) measurement is performed on all multimode fibers to assess modal dispersion from which fiber selection is based. It is not feasible, however, to accurately assess the alpha of a fiber from its DMD plot with enough accuracy for this purpose, since in multimode fiber production multimode fibers do not exhibit a perfect alpha profile and generally exhibit complex DMD patterns.
One approach to this problem includes computing an effective modal bandwidth (EMBc). This computation assesses modal bandwidths only, and thus does not assess the bandwidth resulting from the interaction of the chromatic and modal dispersions. Effective modal bandwidth is computed using a weighted sum of the traces recorded for different offset launches over the whole core radius (i.e., the DMD plot), to calculate temporal responses of VCSEL launches. The weight coefficients are called weight function, and each offset launch corresponds to a given weight. One drawback to this approach is that computed effective modal bandwidth is less and less representative of the power penalty at a given bit error rate as the length of optical link and/or data bit rate of the optical link increase.
In another approach described in European application EP2144096, the DMD pattern is modified to partly account for the modal and chromatic dispersion interaction, and effective bandwidth is then computed with this modified DMD pattern using similar weight functions used for EMBc calculations. This second approach works well when multimode fibers are close to perfect alpha and for simple DMD patterns, however, when multimode fibers start to move away from perfect alpha profiles and/or present too complex a DMD pattern, computed effective bandwidth becomes less representative of the power penalty at a given bit error rate.
In a third approach described in US application US2010/0315620, multimode fibers are selected when they exhibit a negative peak delay difference between a first radius and a larger radius. One drawback to this approach is that there are very sensitive measurements and results that are not representative enough of multimode fiber performances, especially when the DMD pattern becomes complex.